Some of the displacement controlled hydraulic proportional valves have a main control valve for controlling the main fluid flow between a supply pump and a hydraulic motor and a pilot valve for controlling the actuation of the main control valve. The pilot valve is typically controlled by a proportional solenoid exerting a control force on a pilot valve spool to move the pilot valve spool toward a main control valve spool. Moving the pilot valve spool toward the main control spool controls fluid pressure in a control chamber such that the main spool moves toward the pilot valve spool. The displacement of the main control spool is mechanically fed back to the pilot valve spool through a force feedback spring so that displacement of the main spool is proportional to the control force exerted on the pilot valve spool by the solenoid.
One of the problems encountered with such displacement controlled hydraulic proportional control valves is that the main spool has a tendency to oscillate or become unstable under some operating conditions. One of the factors contributing to the instability is the fact that displacement of the main control valve spool is dependant upon the control force and feedback force on the pilot valve spool reaching equilibrium. Thus, if the acceleration forces on the main spool are too high, the main spool tends to overshoot the desired position such that the feedback forces acting on the pilot valve causes the pilot valve spool to oscillate which, in turn, causes the main spool to oscillate. The same pilot valve is frequently used with several sizes of main control valves and the instability is more pronounced on the control valves having smaller diameter main valve spools.
The force feedback spring is normally a coil compression spring and another problem encountered is that the spring tends to buckle under compression. This imposes a side load or torque on the pilot valve spool causing increased friction between the spool and the bore. The increased friction may result in hunting movements of the pilot and main spool.
It would, thus, be advantageous to have a displacement controlled hydraulic proportional control valve design which is stable regardless of the size of the main control valve spool and reduces the potential for hunting movements.
The present invention is directed to overcoming one or more of the disadvantages or problems as set forth above.